Make do_builtin_io multi-fork safe, moved it to postfork.cpp

Addresses https://github.com/fish-shell/fish-shell/issues/495

common.cpp

@@ -105,6 +105,8 @@ static char *wcs2str_internal(const wchar_t *in, char *out);
 
 void show_stackframe()
 {
+    ASSERT_IS_NOT_FORKED_CHILD();
+    
     /* Hack to avoid showing backtraces in the tester */
     if (program_name && ! wcscmp(program_name, L"(ignore)"))
         return;